The results of 1904 allogeneic HLA identical sibling donor bone marrow transplants performed in 52 European centers between 1979 and 1986 and reported to the EBMT leukemia registry were analysed by geographical location of the transplant. Patients were grouped into six regions: United Kingdom, Nordic Group, Benelux, France, Central Europe and Southern Europe. There were significant differences between these regions with respect to patient population and outcome. The relative proportion of the three major disease categories, stage and subtype of the diseases, graft-versus-host disease prevention methods, donor recipient sex combinations, age of the patient, year of the transplant and the time intervals from diagnosis to transplant, from diagnosis to first complete remission for acute leukemia and the time from first complete remission to the transplant varied from region to region. The analysis of outcome parameters showed a significant difference in relapse incidence from region to region. This influence of region was confirmed in a multivariate analysis and was independent of the other factors known to affect outcome. Leukemia-free survival and transplant-related mortality were not different. The reasons for these differences could not be explained by the data in the registry. We conclude that regional factors must be considered when bone marrow transplant data are compared and we postulate that pretransplant factors probably affect outcome more than was previously realized.